1. Field of the Invention
The present invention relates to magnetic data storage systems generally and, more specifically, to disk clock locking using servo marks in magnetic storage systems.
2. Description of the Related Art
In magnetic disk data storage devices (also referred to as hard drives), the data is stored as magnetic flux regions or “magnets” along the surface of a rapidly spinning magnetic media or hard disk with one or more read/write heads “floating” or “flying” very near the media surface. Given the high data rates and low latency requirements of modern hard-drive read channels, data read from the hard disk is sampled and processed using a disk-locked clock (DLC). The DLC is phase-locked to the rotation speed of the hard disk so that the DLC tracks variations in the rotational (angular) speed of the disk when reading and writing to the disk.
There are two kinds of servo tracks on the hard disk. A radial servo track has a conventional (not spiral) servo pattern that contains track information and is used to lock the DLC during writing and reading of user data to and from the disk. A spiral servo track is used to lock the DLC as the radial servo track is written onto the hard disk. A well-known technique for writing the radial servo track is a Spiral Self-Servo Write (SSSW) process that writes the radial servo pattern onto the hard disk using the drive's own read head, write head, and servo system. The drive uses the radial position and timing information of a spiral servo track to write the radial servo track. The DLC system locks to the spiral servo track on the disk to perform the radial servo track write.
One technique for synchronizing the DLC to the radial servo track is to measure a time interval between spiral servo tracks written in a spiral patterns across the disk. The spiral servo tracks are written in such a way that, for a given disk rotation speed, the amount of time between when the spiral servo tracks pass under a read head is the same regardless of the head's radial position over the disk, i.e., how far away the head is from the center of the disk. Thus, for any radial position of the head, the time between spiral servo tracks is the same. In typical embodiments, a counter clocked by the DLC is sampled only once each time the read head encounters the spiral servo track and the value of the counter (a “time stamp”) is compared to the immediately preceding time stamp and any difference in the time stamp values is used to generate a DLC-to-rotational speed phase error to correct or adjust the phase and frequency of the DLC so that it tracks the rotational speed of the disk, thereby phase-locking the DLC to the rotational speed of the disk.
The spiral servo tracks are typically a series of servo address marks (SAMs) separated by a short preamble. The preamble is a repeating pattern (e.g., 2T magnet length having a pattern . . . 11001100 . . . ) and the SAM pattern is a pre-established programmable wide biphase encoded pattern, in one example a nine-bit data pattern of 0001010012 that is biphase-encoded. However, because the servo tracks are placed at an angle with respect to the data tracks that the read head follows over the surface of the disk, the amplitude of the preambles and SAMs as read by the head varies considerably. In a typical hard drive, a series of sequential time-based windows are opened to detect a SAM in the signals from the read head, each window being just long enough for one SAM to occur therein. The SAM in the window having the highest amplitude signal therein is chosen as the SAM used to trigger sampling of the DLC-driven counter value. Knowing which window has the largest signal amplitude might be determinable only after it has passed the head. Moreover, because the window with the largest signal amplitude might not be exactly in the center of the servo track, there might be servo track-to-servo track differences in counter values that are not due to rotational speed variations in the disk. Thus, the differences in the sampled counter values can induce unwanted jitter in the phase of the DLC. It is therefore desirable to have a DLC-to-rotational speed phase error detection technique that is less susceptible to window signal amplitude variation and position of the SAM in the servo track.